This device and method of protection relates to anchoring devices, such as bolts, which are used to install tall, heavy and/or large structures which are subject to high overturning moments. The disclosed device more specifically relates to a grout sleeve for an anchor bolt used in combination with a threaded anchor, and a method for installing the same.
The anchor bolts may either be set in concrete or drilled into the rock. Among other applications, the anchor bolts are used for supporting wind turbines, power line towers, structures used for street lighting and traffic signals, bridge supports, gondola and ski lift support structures, and large signage supports. More particularly, this invention comprises an apparatus, and a method for installing the apparatus, where the apparatus protects the anchor bolts from moisture and corrosive attack caused by water or other liquid entering the annulus formed between the anchor bolt and the anchor bolt sleeve.
The integrity of the foundation of a tall structure is subject to failure if the anchor bolts are not adequately protected. In particular, anchors are subject to corrosive attack caused by the accumulation of water or other electrolytes in the anchoring hole and retained by the anchor bolt sleeve, which can cause a corrosion cell. As described below, the practices employed in preparing the foundation for a wind turbine often create an environment in which the anchor bolt is exposed to water or other liquid.
The initial attempt at solving the anchor bolt corrosion problem was to paint the anchor bolts. However, this solution is labor intensive and does not prevent liquid accumulation around the anchors. In addition, this protection method requires that the anchor bolts be repainted periodically, as well as after re-tensioning the anchor bolts if required in the particular application.
By way of background for wind turbine foundations, U.S. Pat. Nos. 5,586,417 and 5,826,387, both by Henderson, disclose a pier foundation “which can be poured-on-site monolithically and is of cylindrical construction with many post-tensioned anchor bolts which maintain the poured portion of the foundation under heavy compression, even during periods when the foundation may be subject to high overturning moment.” Henderson's foundation is preferably in the shape of a cylinder, having an outer boundary shell and an inner boundary shell each formed of corrugated metal pipe.
In the fabrication of one type of foundation for wind turbines, elongated high strength steel bolts, generally fashioned from 1¼″ (#10) rebar material or 1⅜″ (#11) rebar material, extend vertically up through the concrete from a peripheral anchor plate or ring near the bottom of the cylinder to a peripheral connecting plate or flange at the base of the wind turbine or other structure. The bolts extend through hollow tubes or sleeves to prevent adhesion of the concrete to the bolts. The sleeves are installed prior to delivery of the bolts to the job site, and nuts are generally be placed on each end of the anchor bolt to retain the sleeve on the anchor bolt material.
Henderson further discloses the post-stressing of the concrete in great compression by tightening the nuts on the high strength bolts to provide heavy tension from the heavy top flange (i.e., the flange at the base of the wind turbine) through which the bolts pass to the anchor flange or plate at the bottom of the foundation, thereby placing the entire foundation, between the heavy top plate or flange and lower anchor plate or flange, under high unit compression loading. The nuts on the bolts are tightened so as to preload the bolts to exceed the maximum expected overturning force of the tower structure on the foundation. Therefore the entire foundation withstands various loads with the concrete always in compression and the bolts always in static tension.
The concrete foundation for a turbine tower typically comprises a grout trough which is formed by the placing of a circular template that holds the anchor bolts when pouring the uppermost part of the concrete foundation. Thus, the bottom surface of the grout trough is formed by the top surface of the concrete foundation. The grout trough forms a ring into which high compressive strength grout is poured, where the ends of the anchor bolts extend above the concrete and through the grout poured into the grout trough. Because the tower flange must be set nearly perfectly level, the current practice is to place shims in the grout trough and level the tower flange with laser leveling techniques. Once the shims are leveled, high strength grout is poured into the grout trough and the flange set down on the anchor bolts and the grout allowed to be set up.
Proper alignment of the anchor bolts is important as the anchor bolts must fit within the bolt holes provided in the flange. While there is a slight tolerance for misalignment, in the range of ⅛ to ¼ inches for an individual anchor bolt, the anchor bolts as an anchor bolt package must be nearly perpendicular to the flange and closely matched to ensure a correct and safe tower installation. The ability to align the bolts within the poured and set foundation is helpful to ensure matching the anchor bolts to the bolt holes in the flange.
The anchor bolts may be placed in side-by-side pairs, the pairs extending radially from the center of the foundation, forming an inner ring of bolts and an outer ring of bolts. The bolt pattern is, of course, determined by the bolt pattern on the mounting flange of the structure to be installed on the foundation. A large number of bolts are typically used for this type of foundation. For example, Henderson discloses an embodiment having forty-eight tensioning bolts in the inner ring and forty-eight tensioning bolts in the outer ring for a total of ninety-six. Alternative foundations can utilize more bolts, compounding the problem of anchor bolt alignment with the flange.
In Henderson's foundation, the lower ends of the bolts are anchored at the bottom of the foundation to a lower anchor ring which may be constructed of several circumferentially butted and joined sections. It is to be appreciated that other means may be employed for anchoring the bolts, including drilling a portion of the anchor bolt into the ground.
The bolts usually used for the anchors for wind turbines are approximately thirty feet in length, and usually have outside diameters of 1¼ inch or 1⅜ inch. The hollow tubes or sleeves are typically elongated plastic tubes fabricated from polyvinyl chloride (“PVC”) which encase the bolts substantially through the entire vertical extent of the concrete. The room provided by the PVC sleeves allows the bolts to move under the tension generated by applied pressure, and to be tensioned after the concrete has hardened and cured, thereby post-tensioning the entire concrete foundation. The open ends of the PVC sleeves are generally flush with the top surface of the concrete foundation within the grout trough, thus presenting an opening into the annulus between the anchor bolt and the PVC sleeve.
However, the PVC sleeves do not extend along the entire length of the anchor bolts. Specifically, the PVC sleeves do not extend through the peripheral connecting plate or through the bolt holes in the flange at the base of the wind turbine. The hole diameters of flanges used at the base of wind turbines are approximately 1½ inch, and the external diameters of the commonly available PVC sleeves which may be utilized for 1¼ inch to 1⅜ inch diameter bolts are too large to be inserted within the holes in the peripheral connecting plate or flange.
The inability to insert the PVC sleeves into the flange at the base of the wind turbine creates a problem with respect to preventing the flow of water or other liquids down the annulus formed between the anchor bolt and the PVC sleeve. In the known installations, water may flow into and accumulate in the annulus created by the PVC sleeve and the anchor bolt. The accumulation of water or other liquids can result in the formation of a corrosion cell and cause corrosion in the anchor bolts thus affected. Compounding the problem is that it is a common practice to place water within the grout trough prior to pouring the grout to prevent uneven drying of the grout. However, placing the water in the trough causes it to gravitate into the open PVC sleeve ends which are flush with the top surface of the concrete foundation forming the bottom of the grout trough. Prior art methods of sealing the annulus included wrapping a piece of foam material around the bolt and wrapping the foam sleeve with duct tape to retain the sleeve around the anchor bolt, or running a bead of sealant such as caulking compound around the anchor bolt and grout trough juncture. These are labor intensive processes which are not always successful in preventing the flow of water or other fluids into the anchor bolt/PVC sleeve annulus. Additionally, the foam sleeves are extremely compressed and deformed when the heavy tower flange is set down atop the sleeves. The compressed and deformed foam sleeves displace grout, thus diminishing the overall compressive strength of the grout.